The subscriber identity module (SIM) is widely used in mobile communications. Such a module is used in a mobile communication terminal in the form of a SIM card.
Mobile terminals can be used not only for long-range communications but also for short-range transactions, such as electronic payment. They can serve as a smart card, a credit card, or an access control device.
Different mobile terminals, due to differences in their internal structures and materials used to build these terminals, have a very large variance in their effectiveness of transmitting RF signals. Those mobile terminals that are relatively “transparent” to RF can have an RF communication range of several meters. Those mobile terminals that are less “transparent” to RF may have an RF communication range of only a few centimeters.
In electronic payment applications, such as when using a smart card for public transit fare payment, it is generally required that the transaction range is controlled, such as within 10 cm, to avoid transactions being made to other cards near the card reader without users' knowledge or authorization. Meanwhile, the communication within the transaction range needs to be reliable and fast.
To avoid the large differences in the RF signal attenuations caused by the different mobile terminals, it may be necessary to calibrate the mobile terminals. That is, before adding or using RF functionalities, the attenuation parameters of the mobile terminals must be recorded.
Another mobile payment technology, near-field communication (NFC), is evolved from contactless technologies based on ISO 14443 standards, which adopt a 13.56 MHz magnetic field to transmit both signals and energy. To use this technology, it is necessary to adopt specifically designed mobile terminals or to significantly modify existing mobile terminals.
At the frequency of 13.56 MHz, the card reader and the card utilize the coupling of electrical conduction coils to exchange signals and transmit energy. The card reader is required to simultaneously transmit energy and 13.56 MHz amplitude-modulation signals, and thus there is a stringent requirement for the size and area of the reception coil in the card. In the direction from the card to the card reader, the card relies on short-circuit and open-circuit in-card coil load modulation method, not relying on an external power source, to directly transmit information to the card reader through the transmission field. Due to that the load modulation signal requires better coupling coefficient between the coil in the card and the coil in the card reader, these technologies further have more requirements of the size and area of the on-card antenna. In addition, because of the relatively low frequency of 13.56 MHz, the coupling coil has a relatively large size.
The antenna coil in NFC technologies, by virtue of its large size, cannot be fitted into a SIM card, an SD/TF card, or other cards used for mobile terminals. Furthermore, the metal and other conductive objects in a mobile terminal can interfere seriously with the effectiveness of reception and load modulation. To achieve good communication, therefore, it is necessary to modify a mobile terminal, such as a cellular phone, in a specific manner to optimize the effect of the antenna. Illustrative of such modifications include is the placement of the multi-turn antenna on the back cover of the mobile terminal battery pack and the use of a flexible connection to lead the antenna from the terminal motherboard to the backside of the battery pack. The back cover of the cellular phone also cannot be a metal material.
Even if an NFC antenna can be disposed into a mobile terminal, the signals at 13.56 MHz frequency can form strong eddy currents when encountering metal and other conductive objects. As a result, the signal strength changes following the different structures of the mobile terminals, resulting in a large variations in the field strength on the reception antenna of the NFC card, making it necessary to calibrate for the different mobile terminals.
Due to the above factors, NFC requires the antenna coil in the mobile terminal to be sufficiently large. Such antenna coils cannot fit into the SIM cards, SD/TF cards or other cards used for the mobile terminals. Furthermore, the metal and other conductive objects in a mobile terminal can seriously interfere with the effectiveness of the reception and load modulation. Thus, to achieve a good communication, it is necessary to modify the mobile terminals such as cellular phones in specific manners, to optimize the effect of the antennas. The modifications include, for example, placing the multi-turn antenna on the back cover of the mobile terminal battery pack, or using a flexible connection to lead the antenna from the terminal motherboard to the backside of the battery pack. The back cover of the cellular phone cannot be a metal material.
In Japan, for example, electronic payments using cellular phones have been popular. However, due to the limitations of the ISO 14443 technologies, only specifically-designed cellular phones can be used.
Chinese Patent CN 200810142624.1 proposed a method for controlling an RF communication range for a mobile terminal. Various mobile terminals are tested to obtain their respective RF field distribution characteristics, which are pre-stored in these mobile terminals. An array of detectors is used to measure an RF field distribution adjacent a mobile terminal. The measured field distribution is compared with the pre-stored field distribution characteristics to derive a distance between the RF mobile terminal and an RF control terminal. The derived distance can then be compared with a predetermined range to determine whether the mobile terminal is within a specified communication range. This calibration process is relatively complex, and the RF signals may interfere with it.